The main objective of the proposed research is to study with the light and electron microscope the connections and ultrastructural organization of the superior colliculus, a structure important for controlling head and eye movements. In one series of experiments, anterograde and retrograde axonal transport techniques are being used to identify the cell layers in the superior colliculus which give rise to particular efferent pathways. Once the cells of origin for a particular pathway are identified, several techniques are combined to identify their visual and non-visual inputs. Cells are retrogradely-filled with a mixture of horseradish peroxidase (HRP) and saponin, a substance which increases membrane permeability and HRP uptake. The brain sections containing the backfilled cells are then reacted with ortho-tolidine. This procedure homogeneously fills the soma and dendrites of the cells with an electron-dense HRP reaction product. In the same animal, axon terminals of potential inputs are anterogradely labelled by either degeneration or autoradiography, and the tissue examined with the electron microscope. Labelled terminals can be seen contacting the electron dense processes of the backfilled cells. This approach allows us to determine the inputs to neurons with known efferent projections. In a related series of experiments, colliculus cells are characterized physiologically and then intracellularly injected with HRP in order to visualize in detail their dendritic and axonal branching patterns. Using this combination of techniques, we can study the anatomical basis for the integrative action of the nervous system at the level of the single neuron.